Reference is made to the commonly assigned U.S. patent applications, the disclosures of which are incorporated herein by reference.
U.S. patent application Ser. No. 09/679,016, filed Oct. 4, 2000, in the names of Arun Chowdry et al, entitled DOUBLE-SLEEVED ELECTROSTATOGRAPHIC ROLLER AND METHOD OF USING.
U.S. patent application Ser. No. 09/679,113, filed Oct. 4, 2000,in the names of Robert Charlebois et al, entitled INTERMEDIATE TRANSFER MEMBER HAVING A STIFFENING LAYER AND METHOD OF USING.
U.S. patent application Ser. No. 09/679,177, filed Oct. 4, 2000,in the names of Muhammed Aslam et al, entitled SLEEVED ROLLERS FOR USE IN A FUSING STATION EMPLOYING AN INTERNALLY HEATED FUSER ROLLER.
U.S. patent application Ser. No. 09/680,133, filed Oct. 4, 2000,in the names of Arun Chowdry et al, entitled SLEEVED PHOTOCONDUCTIVE MEMBER AND METHOD OF MAKING.
U.S. patent application Ser. No. 09/680,134, filed Oct. 4, 2000,in the names of Muhammed Aslam et al, entitled SLEEVED ROLLERS FOR USE IN A FUSING STATION EMPLOYING AN EXTERNALLY HEATED FUSER ROLLER.
U.S. patent application Ser. No. 09/680,135, filed Oct. 4, 2000,in the names of Jiann-Hsing Chen et al, entitled TONER FUSING STATION HAVING AN INTERNALLY HEATED FUSER ROLLER.
U.S. patent application Ser. No. 09/680,136, filed Oct. 4, 2000,in the names of Arun Chowdry et al, entitled IMPROVED INTERMEDIATE TRANSFER MEMBER.
U.S. patent application Ser. No. 09/680,139, filed Oct. 4, 2000,in the names of Robert Charlebois et al, entitled INTERMEDIATE TRANSFER MEMBER WITH A REPLACEABLE SLEEVE AND METHOD OF USING SAME.
This invention generally relates to fusing stations and rollers used within electrostatographic imaging and, more particularly, to an externally heated fusing roller having a deformable structure for controlling overdrive and improving image quality.
In electrostatographic imaging and recording processes such as electrophotographic reproduction, an electrostatic latent image is formed on a primary image-forming member such as a photoconductive surface and is developed with a thermoplastic toner powder to form a toner image. The toner image is thereafter transferred to a receiver, e.g., a sheet of paper, and the toner image is subsequently fused to the receiver in a fusing station using heat or pressure, or both heat and pressure. The fuser member can be a roller, belt, or any surface having a suitable shape for fixing thermoplastic toner powder to the receiver. The fusing step in a roller fuser commonly consists of passing the toned receiver between a pair of rollers that are engaged to produce an area of contact pressure known as a fusing nip. In order to form the nip, at least one of the rollers typically has a compliant, or conformable, layer on its surface. Heat is transferred from at least one of the rollers to the toner in the fusing nip, causing the toner to partially melt and attach to the receiver. In the case where the fuser member is a heated roller, a resilient layer having a smooth surface is typically used which is bonded either directly or indirectly to the core of the roller. Where the fuser member is in the form of a belt (as described herein a belt refers to a flexible endless belt that passes around the heated roller) it typically has a smooth, hardened outer surface.
Most roller fusers are of a type known as simplex fusers that function to attach toner to only one side of the receiver at a time. In a simplex fuser, the roller that contacts the unfused toner is commonly known as the fuser roller and is usually heated. The roller that contacts the other side of the receiver is known as the pressure roller and is usually unheated. Either, or both, of the rollers can have a compliant layer on, or near, the surface. In most fusing stations employing a fuser roller and an engaged pressure roller, it is common for only one of the two rollers to be driven rotatably by an external source. The other roller is then driven rotatably by frictional contact.
Another less common roller fuser embodiment known within the prior art, is a duplex fusing station, which has two toner images that are simultaneously attached to each side of a receiver passing through a fusing nip. In such a duplex fusing station there is no real distinction between fuser roller and pressure roller with both rollers perform similar functions in providing heat and pressure.
Two basic types of simplex heated roller fusers have evolved. The first uses a deformable pressure roller to form the fusing nip against a hard fuser roller, such as in a Docutech 135 machine made by the Xerox(copyright) corporation. The second uses a deformable fuser roller to form the nip against a hard or relatively non-deformable pressure roller, such as in a Digimaster 9110 machine made by Heidelberg Digital LLC(copyright). A deformable fuser roller as designated, herein, typically includes a conformable or a compliant layer having a thickness greater than about 2 mm and in some cases exceeding 25 mm. A hard fuser roller as designated herein, typically includes a rigid cylinder which can have a relatively thin polymeric, or conformable, elastomeric coating that is typically less than about 1.25 mm thick. A deformable fuser roller used in conjunction with a hard pressure roller tends to provide easier release of a receiver from the heated fuser roller, because the distorted shape of the deformable surface in the nip tends to bend the receiver towards the relatively non-deformable pressure roller and away from the much more deformable fuser roller.
Conventional toner fuser rollers typically have a cylindrical core member (which is often metallic such as aluminum) coated with one or more synthetic layers (which typically include polymeric materials made from elastomers).
The most common type of fuser roller is one that is internally heated, which means that a source of heat is provided inside the roller to generate the heat required for fusing. Such a fuser roller normally has a hollow core, inside of which is located a heating source, usually a lamp. Surrounding the core is an elastomeric layer through which heat is conducted from the core to the surface, and the elastomeric layer typically contains fillers for enhanced thermal conductivity.
Another type of fuser roller is referred to as an externally heated fuser roller is heated by surface contact between the fuser roller and one or more heating rollers. An example of an externally heated fuser roller is the Image Source 120 copier marketed by Eastman Kodak Company(copyright). Externally heated fuser rollers are also disclosed by U.S. Pat. No. 5,450,183 issued to O""Leary and in U.S. Pat. No. 4,984,027 issued to Derimiggio et al.
A deformable fuser roller can include a compliant or conformable layer of any useful material, such as for example a substantially incompressible elastomer (those having a Poisson""s ratio approaching 0.5). Such a substantially incompressible compliant layer including a poly(dimethyl siloxane) elastomer has been disclosed by Chen et al., in U.S. patent application Ser. No. 08/879,896 which is hereby incorporated by reference. Alternatively, a conformable layer can include a relatively compressible foam having a value of Poisson""s ratio much lower than 0.5. A conformable polyimide foam layer is disclosed by Lee in U.S. Pat. No. 4,791,275. A lithographic printing blanket is disclosed by Goosen et al. in U.S. Pat. No. 3,983,287, illustrating a conformable layer containing a vast number of frangible, rigid-walled tiny bubbles which are mechanically ruptured to produce a closed cell foam having a smooth surface.
Receivers remove the majority of heat during fusing. Since receivers that have a narrower width than the fuser roller width, can have heat removed differentially, causing areas of higher temperature or lower temperature along the fuser roller surface parallel to the roller axis. Higher or lower temperatures can cause excessive toner offset in roller fusers.
Improved heat transfer to the surface of an externally heated fuser roller from external heating rollers will reduce the temperature of the external heating rollers as well as the mounting hardware and bearings attached to the external heating rollers.
A fuser module is disclosed in U.S. Pat. No. 6,016,409, issued to M. E. Beard et al., which includes an electronically readable memory that is permanently associated with the fuser module. This allows the control system of the printing apparatus to read codes from the electronically readable memory during installation to obtain parameters for operating the module, such as maximum web use, voltage and temperature requirements, and thermistor calibration parameters.
When a deformable roller is distorted to form a fusing nip, the thickness of the deformable material on the core is reduced inside the nip. When the deformable layer is substantially incompressible, the average speed of the portion of deformable layer within the fusing nip is inherently greater than parts of the deformable layer well away from the nip, because the volume flow rate of the elastomer is constant around the roller. This results in a surface speed of the deformable roller inside the nip which is different areas far away from the nip. For example, the deformable roller is a driving roller frictionally rotating a relatively non-conformable pressure roller, the pressure roller will rotate faster than if the fuser roller had been non-deformable, a phenomenon known as xe2x80x9coverdrivexe2x80x9d. Overdrive can be expressed quantitatively, as a peripheral speed ratio, measured as the ratio of the peripheral surface speeds far away from the nip. A peripheral speed ratio of unity is equivalent to a condition of zero overdrive.
The speed ratio in a nip is determined principally by an effective Poisson""s ratio of the roller materials, the moduli of the roller materials, the engagement of the rollers, and the drag torque forces of the rollers. The Poisson ratio of high polymers (a polymer with a high molecular weight) approaches 0.5, and approaches zero for very soft polymeric foams. It has been shown in theoretical model computations by K. D. Stack, Nonlinear Finite Element Model of Axial Variation in Nip Mechanics with Application to Conical Rollers [Ph.D. Thesis, University of Rochester, Rochester, N.Y. (1995), FIGS. 5-6 and 5-7, pages 81 and 83] that for a special case of a rigid cylindrical roller coated by a layer of deformable material frictionally driving, with no drag, a nondeformable moving planar element, the deformable material should have a value of Poisson""s ratio of about 0.3 in order to have negligible overdrive, i.e., a speed ratio which is substantially equal to unity. For values of Poisson""s ratio larger than about 0.3, the surface of the roller (distorted by the nip) is stretched in the nip zone, producing overdrive of the planar element with respect to the roller. For values of Poisson""s ratio smaller than about 0.3, the surface of the roller is compressed inside the nip zone, producing underdrive of the planar element with respect to the roller, i.e., the surface speed is smaller than the peripheral speed of the roller far away from the nip.
A foam or sponge can include a xe2x80x9cfeltedxe2x80x9d material, as is well known in the art. Felted foams can be made, for example, by compressing under heat, typically uniaxially, an elastomeric, previously made foam, followed by cooling it under compression and then removing the compressive load. Felted foams have anisotropic mechanical properties such that the Young""s modulus and Poisson""s ratio can be different in different directions. For example, both the Young""s modulus and Poisson""s ratio of a felted foam material made by uniaxial compression will be different along the direction of compression that lies in a plane at right angles to the direction of compression. Moreover, Poisson""s ratio, which tends to be small for soft foams, can even take on negative values in felted foams or sponges.
A deformable roller including a substantially incompressible elastomer that is displaced by distortion in a fusing nip with another roller results in an extra thickness of the deformable layer adjacent to either side of the fusing nip, i.e., pre-nip and post-nip bulges. The highest pressure in the nip will be obtained near the center of the nip (at the intersection of the joined surfaces and an imaginary line between the centers of the two rollers). Since one roller drives the other, the surface velocities of the rollers should be close to equal at the point of maximum pressure, near the center of the nip. As previously explained, the surface velocities in the pre-nip and post-nip bulges will generally be different. In view of these facts, it can be understood that in general there will be locations in the contact zone of the nip where the surface velocities of the two rollers differ, i.e., there will be slippage. This slippage, which can be substantial just after entry and just before exit of the nip, is a cause of wear which shortens roller life.
A potentially serious problem for fusing arising from the presence of overdrive is xe2x80x9cdifferential overdrivexe2x80x9d, associated for example with tolerance errors in mounting the rollers forming the fusing nip, or with roller runout which can be caused by variations of roller concentricity or eccentricity. Runout can have many causes, e.g., fluctuations in layer thicknesses along the length of a roller, variations in the dimensions of a core member, an acentric roller axis, and so forth. It will be evident that differential overdrive can result in localized differential slippages along the length of a fusing nip, inasmuch as the local effective speed ratio would otherwise tend to fluctuate or change with time along the length of the nip, causing some portions of the driven roller to try to lag and other portions to try to move faster than the average driven speed. Differential overdrive can have serious consequences for fusing, including the formation of large scale image defects and wrinkling of a receiver.
All rollers suffer from surface wear, especially where the edges of receivers contact the rollers. Since relative motion due to slippage between rollers increases wear, the changes in velocity of the surface of a deformable roller, as it travels into, through, and out of a fusing nip formed with a relatively non-deformable roller, should increase the wear rate of the deformable roller, especially if the deformable roller is the heated fusing member, bearing in mind that a fuser roller typically faces a relatively rough and abrasive paper surface in the nip. Moreover, since the material on a deformable roller is stretched and relaxed each time it passes through the fusing nip, this flexure can result in fatigue aging and wear, including failure of the roller due to splitting or cracking of the deformable material, or even delamination.
To obtain high quality electrophotographic copier/printer image quality, image defects must be minimized. One type of defect is produced by smearing of image dots or other small-scale image features in the fusing nip. Relative motions associated with overdrive and resulting in localized slippage between rollers in a fusing nip can cause softened toner particles to smear parallel to the direction of motion, resulting for example in elongated dots.
Some roller fusers rely on film splitting of low viscosity oil to enable release of the toner and (hence) receiver from the fuser roller. Relative motion in the fusing nip can disadvantageously disrupt the oil film.
An externally heated toner fuser roller commonly includes a hollow cylindrical core, often metallic. A resilient conformable cushion layer, which can contain filler particles to improve mechanical strength and/or thermal conductivity, is typically formed on the surface of the core, which can advantageously be coated with a primer to improve adhesion of the resilient layer. Roller cushion layers are commonly made of silicone rubbers or silicone polymers such as, for example, poly(dimethylsiloxane) (PDMS) polymers of low surface energy, which minimize adherence of toner to the roller.
Frequently, release oils composed of, for example, poly(dimethyl-siloxanes) are also applied to the fuser roller surface to prevent the toner from adhering to the roller. Such release oils (commonly referred to as fuser oils) can interact with the PDMS in the resilient layer upon repeated use, which in time causes swelling, softening, and degradation of the roller. A thin barrier layer, for example a cured polyfluorocarbon, can be formed on the cushion layer to prevent these deleterious effects caused by release oil.
Electrophotography can be used to create high quality multicolor toner images when the toner particles are small, that is, diameters less than about 10 micrometers, and the receivers, typically papers, are smooth. A typical method of making a multicolor toner image involves trichromatic color synthesis by subtractive color formation. In such synthesis, successive imagewise electrostatic images, each representing a different color, are formed on a photoconductive element, and each image is developed with a toner of a different color. Typically, the colors correspond to each of the three subtractive primary colors (cyan, magenta and yellow) and, optionally, black. The imagewise electrostatic images for each of the colors can be made successively on the photoconductive element by using filters to produce color separations corresponding to the colors in the image. Following development of the color separations, each developed separation image can be transferred from the photoconductive element successively in registration with the other color toner images to an intermediate transfer member. All the color toner images can then be transferred in one step from the intermediate transfer member to a receiver, where they are fixed or fused to produce a multicolor permanent image. Alternatively, an electrophotographic apparatus including a series of tandem modules can be employed, such as disclosed in U.S. patent application Ser. No. 09/199,896, filed in the names of Herrick et al., wherein color separation images are formed in each of four color modules and transferred in register to a receiver member as the receiver member is moved through the apparatus while supported on a transport web.
To rival the photographic quality produced using silver halide technology, it is desirable that these multicolor toner images have high gloss. To this end, it is desirable to provide a very smooth fusing member contacting the toner particles in the fusing station.
In the fusing of the toner image to the receiver, the area of contact of a deformable fuser roller with the toner-bearing surface of a receiver sheet as it passes through the fusing nip is determined by the amount of pressure exerted by the pressure roller and by the characteristics of the deformable material. The extent of the contact area helps establish the length of time that any given portion of the toner image will be in contact with and heated by the fuser roller.
Filler particles can be included in a barrier layer. For example, Chen et al. U.S. Pat. No. 5,464,698, the disclosure of which is incorporated herein by reference, describes a toner fuser member having a silicone rubber cushion layer and an overlying layer of a cured fluorocarbon polymer in which is dispersed a filler including a particulate mixture that includes tin oxide.
Chen et al., in U.S. patent application Ser. No. 08/879,896 disclose an improved fuser roller including three concentric layers each including a particulate filler, i.e., a base cushion layer including a condensation-cured PDMS, a barrier layer covering the base cushion and includes a cured fluorocarbon polymer, and an outer surface layer including an addition-cured PDMS, the particulate fillers in each layer including one or more of aluminum oxide, iron oxide, calcium oxide, magnesium oxide, tin oxide, and zinc oxide. The barrier layer, which can include a Viton(trademark) elastomer (sold by DuPont) or a Fluorel(trademark) elastomer (sold by Minnesota Mining and Manufacturing), is a relatively low modulus material typically having a Young""s modulus less than about 10 MPa, and it therefore has a negligible effect upon the mechanical characteristics of the roller, including overdrive.
To reduce wear and aging and thereby prolong the life of an externally-heated fuser roller for use in electrostatography, there remains a need for a reduced or negligible propensity to exhibit overdrive behavior when the fuser roller is engaged in a fusing nip with a pressure roller. To improve image quality, there remains a further need for an externally-heated fuser roller that has a negligible propensity to produce overdrive-induced image defects, either largescale or small-scale, when used with a pressure roller.
The present invention addresses the above discussed shortcomings within the prior art by providing an eternally-heated toner fusing roller which includes a deformable structure having an effective or operational Poisson""s ratio within a predetermined range. The invention provides an externally heated deformable multilayer fuser roller for improved fusing of toner images in an electrostatographic machine. The individual layers of a preferred fuser roller have Poisson""s ratios and Young""s moduli chosen in preselected ranges, to minimize overdrive and also to minimize differential overdrive in a fusing station. Because of reduced overdrive, a roller of the invention wears slowly and has a long operational life. Moreover, because of reduced differential overdrive, image quality is improved and paper wrinkling is reduced. An overdrive-controlling externally heated fuser roller of the invention can be used in simplex and duplex fusing stations.
In accordance with the invention there is provided a deformable toner fuser roller, for use in a fusing station of an electrostatographic machine including a deformable toner fuser roller for use in a fusing station of an electrostatographic machine, comprising a rigid cylindrical core member; a replaceable removable sleeve member surrounding and in intimate nonadhesive contact with the core member; and wherein the fuser roller is adapted to be heated by a heat source external to the roller.